The present invention relates to a higher definition widescreen TV system compatible with the NTSC system and, more particularly, a method for maintaining the resolution of the luminance signal of a central portion of a screen as much as possible.
As a system which is compatible with the existing NTSC system for transmitting a TV signal for a widescreen, there is proposed "A Single Channel, NTSC Compatible Widescreen EDTV System", by Isnardi et al.
According to this system, as shown in FIG. 2, the central portion CENT of a component 1 having an aspect ratio of approximately 5:3 is transmitted substantially according to the existing NTSC system. The right and left side panels are attached to the small portions at the ends of the existing image such that only their luminance low-frequency components are time-compressed. These attached side portions are exclusively modulated to the NTSC color TV signals and time-expanded about four times (or the band width is dropped to about 1/4) to form a component 3.
These are illustrated in a one-dimensional frequency region in FIGS. 1A to 1D.
In FIG. 1A, letter E designates the well-known NTSC color TV signals corresponding to the two side panels. These color TV signals are time-expanded four times, as indicated at Es in FIG. 1B.
Then, these signals are multiplexed to form a component 2 appearing in FIG. 2. These relations are illustrated three-dimensionally inn FIGS. 3A and 3B. In the NTSC system, as is well known in the art, the color signal is multiplexed on a luminance signal Y.
If a frequency spectrum is illustrated in the three-dimensional dimensional frequency domain with three frequencies, i.e., horizontal .mu., vertical .nu. and time f frequencies, as is disclosed in U.S. Pat. No. 4,660,072 granted to Fukinuki, the color signal C is present in the second and fourth quadrants, and the conjugate first and third quadrants are vacant.
In these quadrants, there may be arranged as luminance signal Ys of the signal Es to be newly multiplexed. For this operation, there may be used a sub-carrier .lambda.o in which a scanning line of identical phase drops for each field, as shown in FIG. 7 in the above-specified U.S. Pat. No. 4,660,072. The luminance signal Ys is illustrated in regions 10 and 10' in FIG. 3B.
In this case, the color components Cs of the signal Es are arranged at regions 11, 11', 11" and 11"'.
Incidentally, the modulated two side panel signals modulate the sub-carrier .mu.o with double side-band modulation as shown in FIG. 1C. This modulation is accomplished for, e.g., multiplexing another signal on that band by, e.g., quadrature modulation. However, this modulation will be omitted because it has no direct relation to the present invention.
The problem accompanying the prior art described above is that the region 12 of the luminance component Y of the main signal (located at the central portion of the image) has to be largely cut off, as illustrated in FIG. 3A, so as to multiplex the double-side-band modulated color signals Cs, as shown in FIG. 3A. In other words, the luminance signal Y for a horizontal frequency .mu. of 1.5 MHz or higher has to be suppressed to about one half of the vertical frequency. The components to be cut correspond to the slant components of the image at the central portion. This cutting process will degrade the image quality seriously.